The present invention relates generally to the field of geophysical exploration and, more particularly, to a method for enhancing geophysical data so as to facilitate the identification and evaluation of subtle variations in the earth's subsurface formations.
In seismic exploration, a source or group of sources is used to impart seismic energy into the earth. A receiver or group of receivers is used to record signals representative of the earth's response to the imparted seismic energy. The recorded signal is generally referred to as a trace or seismic signal. The traces can be used to create a seismic section, i.e., a vertical cross-section of the subsurface of the earth.
Various techniques are used to filter or suppress "noise" (i.e., reception of portions of the imparted energy and other unwanted energy receptions) in the seismic signals that do not represent true reflections from geological features. One such technique is to combine seismic signals or traces from a common depth point (CDP) gather. This combining of traces is known as CDP stacking or summing. In geophysical exploration, displays of CDP stacked seismic signals are used to map the earth's subsurface formations. In order for such displays to convey credible information about the earth's subsurface formations, reflection events in the seismic signals must be visible.
A common problem in CDP stacking of seismic signals is that the polarity of the amplitudes of one or more of the reflection events, in the seismic signals representative reflection from a common point, can change from one seismic energy travel path to another (e.g., as between the seismic signals created by different source-receiver pairs). When this happens and the seismic signals are stacked, the reflection event amplitude can be reduced and may even disappear from the CDP stacked signals, i.e., the opposing polarities of reflection event amplitudes tend to cancel in the summation process.
In areas where the polarity of reflection event amplitudes change as a function of source-receiver separation distance, proper identification and evaluation of subtle changes in the earth's subsurface formations associated with such reflection events can be very difficult. Consequently, there exists a need for enhancing geophysical data whereby ordered gathers of seismic signals or traces can be combined without reducing or cancelling the amplitudes of reflection events. The present invention provides a novel method of processing geophysical data responsive to such need as more fully discussed below.